There has been a technique that enables to drive a gas turbine with coal gas obtained by gasifying coal, thereby generating power. To gasify the coal, a coal gasifier is used. When the coal is gasified, slag is left as burnt embers in the coal gasifier. This slag needs to be discharged from the coal gasifier. Because the slag has fluidity when it has a sufficiently high temperature, the slag is generally discharged continuously from a slag hole provided in a lower part of the coal gasifier. A slag discharge tube filled with cooling water is provided below the slag hole, so that the slag is cooled by the cooling water and solidified, and then discharged from the slag discharge tube.
It is important in the operation of the coal gasifier to avoid such a situation that the slag hole is blocked by solidified slag or the flow of the slag becomes unstable. Therefore, to operate the coal gasifier normally, the discharge state of the slag needs to be monitored. For example, Patent Literature 1 discloses a method of monitoring molten slag generated in a gasification fusion furnace. In this method, molten slag flowing down from a slag discharge port is imaged, and when a plurality of separated or branched portions are confirmed in a lower part of the slag flow extracted from the image, it is determined that deposited and solidified slag is generated, which may block the slag discharge hole, so that a solidified-slag removing unit is operated.